NEW ORLEANS -- Former New Orleans Mayor Ray Nagin has been served with a subpoena and ordered to appear before a federal grand jury on Friday, the WDSU I-Team has learned.

Reached on Monday, attorney Robert Jenkins declined to confirm the information. Nagin could not be reached for comment.

The subpoena was delivered last week, the I-Team reported.

A federal grand jury is tasked with determining whether there is probable cause to believe that someone has committed a crime. The Office of the U.S. Attorney -- in this case, led by Jim Letten -- presents evidence and can compel people to testify as it investigates.

Grand jury proceedings are closed, and may produce an indictment or bill of information -- or no action against a subject.

The subpoena follows reports six months ago that Nagin was under formal investigation by federal authorities. Though neither the FBI nor U.S. Attorney's Office would discuss the probe, those close to the matter told WDSU that the investigation focused largely on private business dealings with several city contractors who have either been convicted or pleaded guilty in federal court to charges such as bribery and conspiracy.

The Nagin family in 2004 founded Stone Age LLC, a company headed by Nagin's sons that supplied and installed counter tops in association with several area Home Depot stores. The subpoena indicates that business will be part of the grand jury focus.

In February, the Times-Picayune reported that the federal government could be looking into Nagin's relationship with businessman Aaron Bennett and former City Hall Technology Director Greg Meffert. Bennett has pleaded guilty to bribing former Plaquemines Parish Sheriff Jiff Hingle, and Meffert in June pleaded guilty to accepting kickbacks while on the City of New Orleans payroll.

Also in June, businessman Frank Fradella pleaded guilty to a charge that he bribed a public official. Federal prosecutors said Fradella paid an unidentified individual $50,000 in bribe money.

Though the recipient was not named, the description and dates of the transaction included in federal documents suggested that the public official was Nagin.

The U.S. Attorney's Office noted that Fradella also provided "numerous truckloads" of free granite to "Public Official A," and signed the official to a consulting contract with monthly payments of more than $10,000 following the individual's exit from office.

Letten would not identify Public Official A as he and his team left court on June 27.

"Everybody's got a prediction," he said. "The only people who control that are the prosecutors and the U.S. grand jury.

"We simply follow the evidence fairly and thoroughly, and if it's appropriate, at some time, we'll seek charges against other folks."

Nagin has not been charged with any federal or state crimes, but one local legal analyst suggested the development marked the start of a new chapter in the former mayor's battle with the federal government.

"We have known for some time that he is the end game for the government, so this is the beginning of his legal battle with the feds," attorney Jason Williams said.

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